The present invention relates to a light-emitting organic thin film electroluminescent device usable for a flat light emission panel and display.
The organic thin film electroluminescent device is attracted as a self-light-emitting plane display. The organic thin film electroluminescent device has laminations of organic layers which have different functions for the purpose of remarkable improvement in emission efficiency. The organic thin film electroluminescent device may show a high brightness of luminescence by a voltage which is slightly less than 10V. This organic thin film electroluminescent device is, for example, disclosed in Applied Physics Letters Vol. 51, 1987, p. 913 and Vol. 56, 1990, p. 799. The most of the organic thin film electroluminescent devices has a multi-layered structure of anode/hole transport region/electroluminescent emitter region/electron transport region/cathode. Any one or both of the hole transport region and the electron transport region might not be provided. The hole transport region may comprises a first hole transport layer which allows a hole injection from the anode into the electroluminescent emitter region, and a second hole transport layer which serves as a blocking layer against electrons and excitons supplied from the electroluminescent emitter region.
The hole transport region of the conventional organic thin film electroluminescent device may mostly comprise an aromatic amine formed by evaporation. Particularly, 1,1'-bis(4-diparatrilaminophenyl)cyclohexene or N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine are preferable for materials of the evaporated film due to those excellent hole injection property and good film growth property. Those evaporated film is uniform immediately after the evaporation. After a few days, however, cohesion may appear. This cohesion deteriorates the properties of the device. This is disclosed in Applied Physics Letters, Vol. 68, 1996, p. 1787.
In Japanese laid-open patent publication No. 57-51781, it is disclosed that a porphyrin compound is used for the hole transport layer in the organic thin film electroluminescent device. After the organic thin film electroluminescent device has been formed, then the hole transport layer shows crystallization and cohesion which causes deterioration in property of the device.
In Japanese laid-open patent publication No. 7-110940 as well as Applied Physics Letters Vol. 65, 1994, p. 807, it is disclosed to use ternary amine of Star-Burst type for the hole transport layer. It is, however, difficult to obtain a sufficiently long durability of highly bright luminescence.
In Japanese laid-open patent publication No. 63-295695, it is disclosed that, in order to reduce pin holes for improvement in stability of the organic thin film electroluminescent device, the hole transport layer has a double-layered structure of a first layer of porphyrin based compound for allowing hole injection from the anode and a second layer of aromatic ternary amine. It is, however, difficult to obtain a sufficient improvement in stability of the organic thin film electroluminescent device.
As described above, the organic thin film electroluminescent device shows a high brightness of luminance but has a shorter life-time than other luminescent devices. This short life-time makes it difficult to make the organic thin film electroluminescent device practicable.
In the above circumstances, it had been required to develop a novel organic thin film electroluminescent device having a longer life-time than the conventional ones for enabling the organic thin film electroluminescent device practicable.